You can ping the site. Considering win envirorment and example is if cio.com, then get into dos prompt by typing cmd and ping cio.com, you will get 3 lines of results. In the result if you get results like host not reachable, then the server of the site is down. If the server can be reached then the site is down. If the ping takes a long pause to give a negative result you net is down. You can try a couple of sites to ping and confirm.